The present invention relates to printers, and especially to inkjet printers.
A problem with such printers is that, when an absorbent print medium such as paper is being used, the medium has a tendency to move out of its plane. This movement up from the printing zone is caused by local expansion of the medium as it absorbs the printing ink and creates ruffles or cockle in the medium. Such movements cause variation of the pen-to-paper spacing (PPS) and have an adverse impact on image quality.
Another problem with such printers, which also affects PPS, is the creation of so-called xe2x80x9cwormsxe2x80x9d due to the interaction of the print medium with paper-motion mechanisms. Print medium drive members, such as pinch wheels and overdrive wheels, act on the print medium at specific locations spaced along the scan axis of the printer. Thus they determine different boundary conditions at different locations across the print medium and this effect causes the medium to have a tendency to create ruffles at preferred scan-axis locations. The boundary conditions do not change with time and the resulting variations in PPS affect dot placement so that, in graphics applications, clearer areas or tracks (known as xe2x80x9cwormsxe2x80x9d) are observed extending along the paper axis. Such an impact on image quality is not acceptable.
The present invention seeks to overcome or reduce one or more of the above problems.
According to a first aspect of the present invention, there is provided a printer comprising a printhead and a print medium drive system for advancing a print medium past the printhead, the printhead being arranged to print on the medium in a plurality of successive print medium positions, characterised in that at least one component of the drive system is configured to exert different mechanical forces on the print medium in successive print medium positions.
The component may comprise one or more rollers or wheels with a non-circular periphery.
The component is preferably an overdrive roller arrangement.
According to a second aspect of the present invention, there is provided a print medium roller arrangement characterised in that it comprises one or more rollers having a drive surface with a non-circular periphery.
Alternatively the component may comprise a platen extending along the print axis and incorporating a plurality of suction holes, the platen incorporating a mechanism that selectively closes, at least partially, some of the holes.
According to a third aspect of the present invention, there is provided a printer platen comprising a plurality of suction holes and means for controlling the suction pressure applied by said holes, characterised in that said suction control means are arranged to apply different pressures to different ones of said holes at successive times.